So A123 Systems went public last week. Which green technology startup is going to be next?

"There are a half-dozen companies that I think will meet those criteria that I think could go public in the next 12 months," said Stephan Dolezalek, Managing Director at VantagePoint Venture Partners at the Renewable Energy Finance Forum-WEST in San Francisco. "But I don't see this as a window opening that will allow 30 to 40 companies to go through."

Dolezalek didn't say which startups he saw as IPO-ready. But speaking after his presentation, he did pass on some names he said he's heard others mention.

But the public markets are still reeling from the credit meltdown and global recession that began last year. Given the shaky state of the markets, the risks of an unsuccessful greentech IPO spooking potential investors in other companies is a real concern, they said.

"The coast is not clear," said Neil Auerbach, managing partner with Hudson Clean Energy Partners. "Short sellers are waiting... to see these companies fail. We have to pray for their success."

That's held up so far this week – A123 shares stood at $21.32 at the end of the day Wednesday. It's the biggest greentech IPOs since First Solar went public in 2006.

Silver Spring and Trilliant have contracts to network millions of smart meters and other smart grid devices. Silver Spring has said it expects to become profitable this year, and has hinted at an IPO in the next year or two (see Green Light post).

Bridgelux has been shipping light-emitting diodes for commercial , and earlier this year said that advances in cutting the costs of its LEDs could bring the energy-sipping lights within the reach of consumers (see New LED Aims to Whack Prices of Solid State Lighting).

If a company can't go public, it can seek an exit in being bought, of course. But Pascal Levensohn, founder of Levensohn Venture Partners, said that's an inferior choice, not just for companies concerned but the economy at large.

"When companies are acquired instead of going public, jobs are lost, not created," he maintained. Corporate synergies equal layoffs, he said. But the percentage of IPO's versus acquisitions as exits for U.S. startups has been shrinking, he said – while more than half of startups went public in the 1990s, only 13 percent did so this decade, he said.

And the longer timelines for companies to go public – up to an average of 10 years as of 2008, compared to about four years back in 1998 – make it harder for venture capitalists to earn a return on their investments, he said.

That means that VCs may well have to put themselves in the place of public markets to bring companies past the early technology-proving stages they're used to funding, Dolezalek said.

"We have to get these companies further towards maturity," he said. Of course, that means that traditionally rich venture capital returns for earlier investments at better valuations may be diluted by the lower valuations that come with later-stage investments, he noted.